1. Field of the Invention
The present invention relates to an image forming apparatus for producing copy images by an electrophotographic process or the like, and more particularly to the control of a plurality of developing units within the image forming apparatus.
2. Description of the Prior Art
Image forming apparatus, such as electrophotographic copying machines and reader-printers recently available, include those which are provided with two developing units arranged around an electrostatic latent image bearing member. One of the developing units is used for normal development (development for preparing a positive copy image from a positive original, hereinafter referred to as "P--P development"), and the other developing unit for reverse development (development for preparing a positive copy image from a negative original, hereinafter referred to as "N-P development"). Alternatively, one developing unit is filled with a developer containing a black toner, and the other developing unit with a developer containing a toner of other single colors such as red, yellow or blue. One of the developing units is selectively used for development.
However, the image forming apparatus of the type described has the problem that the toner (especially an insufficiently charged toner portion) is released from the developing station for the unit being used for development and becomes mixed with the developer in the other unit to produce fogged copy images when the other unit is thereafter used. This problem arises because the developing stations for the developing units are arranged adjacent to each other in the direction of rotation of the latent image bearing member, permitting the toner scattering from the developing station of the operating unit to intrude into the developer on the outer periphery of the developing sleeve of the other unit.
As a technique for removing the toner intruding into the other developing unit, it is already proposed to apply a bias voltage to an electrically conductive roller disposed within the second developing unit and electrostatically attract the mixed toner for separation and removal as disclosed, for example, in Unexamined Japanese Patent Publication No. SHO 54-104834.
According to the proposal, however, the toner is transported from the location of intrusion (developing station) to the separation removal position by the rotation of the developing sleeve or magnetic roller and is therefore mixed with the developer to a greater extent during the transport. The extraneous toner once mixed is very difficult to separate and remove.
On the other hand, various means appear useful for preventing the scattering of the toner, but means still remain to be provided for complete prevention.